The present invention relates to a punch rivet for joining two workpieces, said punch rivet having a head and a shank, wherein the shank is realized as a hollow shank with a shank internal diameter, a shank external diameter and a shank end face, wherein on the shank end face a ring cutting edge is realized, the diameter of which is smaller than the shank external diameter and wherein the shank internal diameter merges into the ring cutting edge by means of a cutting radius.
A punch rivet of this type is known from document DE 10 2005 052 360 B4 and from document DE 10 2009 039 936 A1.
The invention additionally relates to a method for producing a punch-riveted joint between at least two workpieces, one of which consists in particular of a high-strength material with a strength of at least 600 MPa and preferably of a maximum of 1000 MPa and comprises in particular a thickness of at least 0.5 mm. In addition, the present invention relates to a workpiece arrangement produced from at least two workpieces which are joined by means of at least one punch rivet of this type or by using a punch riveting method of this type.
Punch riveting is a transformative joining process and can be carried out using so-called solid rivets and so-called semi-hollow rivets. The present case is concerned with punch riveting using a semi-hollow rivet which is realized with a hollow shank. Said method is characterized in that the semi-hollow rivet is driven into an arrangement of two workpieces (in particular two metal sheets), the uppermost workpiece being pierced (perforated) by the hollow shank. A die on which the bottommost workpiece rests, is designed such that the hollow shank is hereafter radially expanded and is driven radially into the bottom workpiece such that an undercut is formed. In this case, the bottom workpiece is as a rule not severed such that the side of the bottom workpiece facing the die remains closed and as a result is less susceptible to rust. Said punch riveting method with a semi-hollow rivet has been used for some years more and more in the automobile industry. The punch riveting method is suitable in particular for joining workpieces produced from different materials (for example steel and aluminium). As there is a trend in particular in car body construction toward a combination of different materials adapted to the respective intended purpose, the punch riveting method is a joining method preferred for this case in comparison to welding.
It is obvious in this case that at least the material of the bottom workpiece on the die side should have good cold forming properties and should have material characteristics that are as homogeneous as possible.
As in the above-described field of application, there is a trend toward high-strength materials (for example strengths of the upper workpiece on the punch side of up to 1000 MPa), the punch rivets used for this purpose have to have a correspondingly high strength (rivet hardness), in particular in order to be able to pierce such a workpiece produced from a high-strength material. At the same time, the punch rivet is to have adequate deformability in its shank region in order to provide a sufficient undercut in the finished punch-riveted joint. Further criteria when punch riveting high-strength metal joints are a sufficiently high level of rigidity for penetrating such high-strength metal sheets as well as a sufficiently large shank interior volume in order to receive the punch slugs from the metal sheet layer on the punch side.
In the field of semi-hollow rivets there are different standard models, for example the so-called C rivet, as is known from document WO 95/09307. The semi-hollow rivet described there comprises a cylindrical hollow shank, a sharp ring cutting edge being realized on the exterior edge of the end face of the shank. For this purpose the internal diameter merges into the shank external diameter by means of a cutting radius. Said C-rivet was originally developed for pure aluminium joints or combinations of low strength steel plates. However, the C-rivet is less suitable for punch riveting high-strength workpieces as the C-rivet has a tendency to expand strongly during the punch riveting operation such that there is only a little material present in the undercut region of the bottom workpiece. The obtainable strength is consequently relatively small. A modified C-rivet with a sharp ring cutting edge which is, however, at a spacing from the shank external diameter is known from document DE 203 19 610 U1.
In addition, a so-called P-rivet is known in the prior art, for example from document DE 10 2005 052 360 B4.
In the case of said punch rivet, a ring cutting edge is realized on the shank end face, the diameter of which is smaller than the shank external diameter. The ring cutting edge is realized as a flat ring surface which is aligned in the radial direction and merges into the shank external diameter by means of a shank external bevel and as a result is at a spacing from the shank external diameter. The shank internal diameter merges into said ring surface by means of a relatively small radius.
The P-rivet certainly has better characteristics than the C-rivet when punch riveting high-strength workpieces. Nonetheless, there is still a need for improvement in this regard even with the P-rivet.
As a result of the increasing pressure on the automobile industry to make noticeable reductions in the CO2 emissions, lightweight construction produced from high-strength and highest-strength metal sheets with corresponding sheet thicknesses and the reductions in weight that are achievable as a result become more and more important.
The semi-hollow punch rivet which was made known from document DE 10 2009 039 936 A1 mentioned in the introduction, is generally suitable for joining high-strength sheet layers. Nevertheless, there is generally a need for improvement with regard to the known semi-hollow punch rivets.